1. Field of the Invention
The present invention relates generally to a switch configuration for key-entry operation and, more particularly, to a key-entry switch device (hereinafter referred to as a key switch) preferably used for a keyboard incorporated, as an input device, in electronic equipment. The present invention also relates to a keyboard provided with a plurality of key switches.
2. Description of the Related Art
In a thinner or a low-profile-type keyboard for a portable electronic apparatus, such as a notebook-type personal computer, it is required to reduce the height of a key switch for a key-entry operation as well as to improve the operability of the key switch.
As a key switch device for such a low-profile-type keyboard, for example, Japanese Unexamined Patent Publication No. 9-190735 discloses a key switch device including a base plate, a key top arranged on the base plate, a pair of link members interlocked to each other to support the key top above the base plate and direct the key top in a vertical or up-and-down direction, and a switch like a sheet (hereinafter referred to as a membrane switch sheet) capable of opening and closing a contact of an electronic circuit in accordance with the vertical or up-and-down movement of the key top. The pair of link members are assembled together into a reverse V-shape as seen in a lateral direction (or in a side view) and meshed at toothed end regions formed on one ends thereof with each other such that the pair of link members may pivot relative to the key top. Further, the pair of link members slidably engage with the base plate at the other ends thereof.
When the pair of link members pivot at one ends thereof in the opposite direction each other such that the other ends thereof are horizontally moved along the base plate, the key top is movable substantially in the vertical direction relative to the base plate, while keeping a predetermined posture of the key top.
In the above key switch device, the key top is moved in the vertical direction while the key top is parallel to the base plate. FIGS. 16a-16c are views explaining the motion of a key top 714 in the vertical direction of a key switch device 710. When the key top 714 is positioned at an uppermost position as shown in FIG. 16a, each sliding portion 746 of a pair of link members 716 contacts one end portion of each groove 750 of guiding sections 748. Therefore, as the link members 716 can not be moved in the sliding direction (in the horizontal direction in FIG. 16a), the key top 714 can not be also moved in the horizontal direction. When the key top 714 is positioned at a lowermost position as shown in FIG. 16c, each sliding portion 746 contacts another end portion of each groove 750, the key top 714 also can not be moved in the horizontal direction.
However, when the key top 714 is between the uppermost and lowermost positions as shown in FIG. 16b, each sliding portion 746 may be moved in the horizontal direction (as indicated by arrows in FIG. 16b) in each groove 750. Therefore, in the state of FIG. 16b (i.e., while the key top is being pressed), the key top 714 may be somewhat displaced in the horizontal direction. This may cause the instability or a slipping movement of the key top, resulting in the deterioration of the operability of the key top.
On the other hand, the key top is firmly held in the direction perpendicular to the drawing, by the stiffness of the link members, during the vertical motion of the key top. However, the displacement of the key top in the horizontal direction is restricted only by the elasticity of an actuating member made from an elastic body such as a rubber arranged between the key top and the membrane switch sheet. Therefore, the key top can not be sufficiently held in the horizontal direction.
In order to resolve the above problem, for example, Japanese Unexamined Patent Publication No. 2001-283676 discloses a shaft of a link provided with a projection having an oval cross section and an opening, with which the projection may be slidably engaged, having one end positioned above the other end. In this constitution, the shaft having the oval cross section is slidably engaged with the opening having the height which varies depending on the horizontal position, whereby a key top cannot bounce or cannot be displaced in the horizontal direction. Therefore, it is necessary to manufacture the projection and the opening with high dimensional accuracy. However, due to wear between the projection and the opening, the gap between them is gradually increased, whereby the effect for avoiding the displacement of the key top in the horizontal direction may be reduced. Further, as the oval cross section of the projection is relatively small, the projection may be easily damaged due to the low stiffness thereof.
Further, Japanese Unexamined Patent Publication No. 2003-92041 discloses a key switch device including a wall member abutting against a pinching axle of a link member. In the device, the pinching axle and the wall member cooperate so as to limit the displacement of a key top in the horizontal direction. However, both of the pinching axle and the wall member are relatively small projections and, the contacting area between them is considerably small. Therefore, also this constitution, similarly to the case of Japanese Unexamined Patent Publication No. 2001-283676, is somewhat insufficient from the viewpoint of the stiffness and the wear property of each component.